Sub-zero treatments in which steel materials such as machine components are cooled to low temperatures of 0° C. or lower are used conventionally to improve the performance such as the hardness and the toughness of the steel materials.
One known sub-zero treatment method is a low-temperature atmospheric method in which the atmosphere inside a cooling tank housing a cooling target object is cooled using a refrigerator or liquid nitrogen or the like, thereby cooling the cooling target object.
Patent Document 1 discloses a sub-zero treatment device (see FIG. 12) used in performing the above sub-zero treatment.
FIG. 12 is a cross-sectional view illustrating a schematic outline of a conventional sub-zero treatment device.
As illustrated in FIG. 12, a sub-zero treatment device 100 disclosed in Patent Document 1 has a cooling tank 102, a refrigerant inlet passage 103, a liquid refrigerant inlet valve 104, a temperature controller 105, an agitation fan 108, and a baffle plate 109.
The cooling tank 102 is formed from an insulating material, and has an internal treatment space. An exhaust port 102A is provided in a side wall of the cooling tank 102, and penetrates through the side wall. When the pressure inside the cooling tank 102 increases due to evaporation of the liquid nitrogen (the liquid refrigerant), the exhaust port 102A externally exhausts a portion of the nitrogen gas from inside the cooling tank 102 to a location outside the cooling tank 102 in order to ensure that the pressure is maintained within a prescribed pressure range.
The refrigerant inlet passage 103 is connected to a liquid nitrogen supply source not shown in the drawing. When the liquid refrigerant inlet valve 104 (the valve provided in the refrigerant inlet passage 103) is opened, the refrigerant inlet passage 103 supplies liquid nitrogen into the cooling tank 102.
The temperature controller 105 measures the temperature inside the cooling tank 102, and adjusts the degree of opening of the liquid refrigerant inlet valve 104 based on the result of the measurements.
The agitation fan 108 is housed inside the cooling tank 102. The agitation fan 108 converts the liquid nitrogen to a mist and diffuses the mist through the interior of the cooling tank 102, as well as agitating the low-temperature nitrogen gas (low-temperature gas) inside the cooling tank 102.
The baffle plate 109 is housed inside the cooling tank 102, and is disposed between the agitation fan 108 and the cooling target object 101. The baffle plate 109 has a suction port and blowout ports. The baffle plate 109 has the function of enhancing the agitation action of the agitation fan 108.